The invention relates to the manufacture of side draft particle classifiers.
Particle classifiers separate particulate solid material into coarse and fine fractions by size and other particle properties such as density. Air classifiers sometimes called air separators are widely used, for example, in the cement industry to recover fine particulate material from raw crushed stone.
One of the older forms of air classifiers known as an updraft classifier is represented by the whirlwind air separator marketed in various forms for over 50 years by Sturtevant, Inc. of Boston, Mass., formerly the Sturtevant Mill Company. Many of the fundamental features of this design are described in U.S. Pat. No. 1,933,606 to T. J. Sturtevant issued Nov. 7, 1933. In the updraft machine, the principle air direction in the critical separation zone is vertically upward. Raw material is dropped onto a spinning distributor plate on the end of a suspended shaft inside an inner casing mounted within a spaced outer casing. The shaft also supports an internal fan above the inner casing which draws air upward through the separation zone entraining fine material and sending it down the annular space between the inner and outer casings. The heavier coarse material falls into a lower tailings cone beneath the inner casing.
As described in U.S. Pat. No. 4,551,241 issued Nov. 5, 1985 assigned to Sturtevant, Inc., the side draft classifier operates in a fundamentally different manner from the traditional updraft whirlwind classifier. In the side draft classifier, an external air source creates a radially inward rather than upward draft through a cylindrical curtain of falling raw material. The inrushing radial draft blows the fine material into a centrally located, spinning, open-bottomed rejector cage. A stationary fines collection chamber is mounted below the rejector cage. The fine material is withdrawn from the fines chamber through a duct or ducts formed through the side wall of the coarse hopper.
In comparison to the older updraft whirlwind separator, the side draft classifier achieves much higher efficiency. The side draft machine combines ultra sharp cuts with low energy consumption and low abrasion in a compact simply operated unit. Narrower particle size distribution of the product results in fewer ultra fine particles and less residue. For cement applications, controlled particle size results in higher compression strengths obtained at lower specific surface areas (cm.sup.2 /g Blaine). Because of the superior design of the side draft classifier, sharp classifications are obtained with lower power consumption. Because of the higher efficiency of the side draft machine, a smaller unit of lower overall weight can be specified instead of a larger, heavier conventional whirlwind updraft separator for the same application. The newer technology increases the flexibility of the installation because of space and weight reduction and reduces the load on the supporting structure. Since the installation and structural support for the classifier represent a major fraction of the cost of the classifier, sometimes exceeding 50% of the cost of a new classifier, the side draft classifier enjoys a significant advantage in new installations because of its reduced structural requirements and greater efficiency.